It's a trailer with possibly one of the most unspeakable scenes in movie history - the first glimpse of Dumb and Dumber To is out and the bar is joyously low.

Twenty years since they first graced the big screen, dim-witted best friends Lloyd Christmas and Harry Dunne are back. Jim Carrey and Jeff Daniels reprise their roles as the two hapless halfwits and the Farrelly brothers are also back behind the camera.

The trailer was launched on The Tonight Show with Jimmy Fallon earlier this week, with Carrey telling the TV host that "it's not a movie anymore, it's a phenomenon."

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In the trailer, Carrey's Lloyd has been locked up in an asylum for two decades and appears to be some kind of catatonic caveman. But when Harry comes to visit, he reveals the lengthy affliction has been a practical joke.

Nonsense ensues, from catheter jokes to a cat called butthole and the aforementioned unspeakable scene with a grandmother that has all the hallmarks of a Farrelly gross-out spectacular.

From there, the thick pair try to track down a young daughter Harry never knew he had. Cameos include Jennifer Lawrence and Kathleen Turner.

Dumb and Dumber To will be released in the US November, with no word as yet on an Australian date.

The original Dumb and Dumber, the Farrelly brothers' directorial debut, came out in 1994 and scored $250 million in box-office receipts worldwide. A 2003 prequel, Dumb and Dumberer: When Harry Met Lloyd, which had neither Carrey nor Daniels or the Farrellys on board, tanked.